The present invention is concerned with protecting steel surfaces from erosion and corrosion.
Steel surfaces when exposed to hot moist steam are particularly vulnerable to attack and deterioration by erosion and corrosion. This is an espcially serious problem in large plants such as nuclear power plants. Accordingly, it is desirable to protect such steel surfaces to at least retard erosion and corrosion effects.
Protective coatings to be efficient and effective for such purpose must satisfy a number of conditions. In particular, to be efficient a protective coating must have sufficient hardness and sufficient chemical resistance. Further it must have sufficient toughness to be able to resist the thermal stresses. Also, it must satisfactorily bond to the base material (e.g., the steel surface).
The protective coating should also form a layer which is sufficiently tight and substantially free from pores so as to prevent chemically active materials from penetrating into the base material.
As far as the above-discussed properties are concerned, the ceramic coating described in the above-mentioned Swedish patent application is in many respects satisfactory. However, the toughness of the ceramic coating is not sufficient since the material is so brittle that it may be destroyed by drip erosion in exposed places, with resultant risks of corrosive attacks on the damaged spots.
A metallic coating with sufficiently high normal potential could, in principle, fulfill all conditions. However, since such a coating must have a relatively high normal potential in relation to the base material, relatively slight damage or even pores in the layer may cause serious galvanic corrosion on the base material.
It is therefore an object of the present invention to provide a protective coating for steel surfaces which avoid the above drawback of using a metallic coating with a relatively high normal potential.